Three-photon absorption and Kerr nonlinearity in pristine and doped β-Ga2O3 single crystals

We report the spectral dependence of the three-photon absorption coefficient γ and third-order nonlinear refractive index n2 in pristine, Sn-doped and Fe-doped β-Ga2O3 crystals using 190 fs pulses in the range of 540–750 nm, i.e., photon energy between half and one-third of the bandgap Eg. The nonlinear coefficient n2 is found to be independent of Sn or Fe doping, and the maximum value of n2 is about 4 × 10−19 m2/W when the photon energy is near Eg/2. However, we observe that Fe doping can significantly enhance the γ value in the range of 540–600 nm (near Eg/2) while doping has negligible impact on γ for wavelength larger than 620 nm. Femtosecond pump-probe measurements show that Fe doping induced a fast (∼50 ps) carrier decay process after photo excitation, which could be attributed to the Fe induced deep trap states. We obtain three-photon figures of merit that allow operational intensities up to 100 GW/cm2. Our results indicate that β-Ga2O3 has great potential for all-optical switching applications in red and near IR regimes.